Antenna sidelobe interference canceller

ABSTRACT

Interference signal cancelling is provided by a multiplexed interference  celling system that utilizes a primary high gain antenna and an auxiliary antenna pair including a low gain notch antenna in conjunction with a low gain omnidirectional antenna. The output of the two low gain auxiliary antennas are subtracted from each other to provide an output that is substantially free of all input signals except for the interfering signal received in the narrow notch of the notch antenna. This signal is then subtracted from the composite output signal of the primary antenna. One of these auxiliary antenna pairs is employed for each separate jamming signal source to be cancelled, thus enabling each jamming signal to be orthogonally isolated, whereupon its magnitude and phase is then adjusted to be equal to and 180° out of phase with the same component contained in the composite signal from the primary antenna. Each interfering signal is then separately subtracted from the composite signal, leaving only the desired signal.

This is a Statuatory Invention Registration (SIR) application of aninvention made by an employee of the U.S. Government, Department of theArmy.

CROSS REFERENCE TO RELATED APPLICATION

This invention is related to the invention shown and described in U.S.SIR application Ser. No. 374,123 entitled, "Auxiliary AntennaInterference Canceller", filed by the same assignee in the name of thesubject inventor on June 13, 1989.

FIELD OF THE INVENTION

This invention relates to electrical antenna systems for communicationsand more particularly to an adaptive antenna system which enablesundesired interference to be cancelled from the incident radiationreceived.

BACKGROUND OF THE INVENTION

One of the major concerns of designers of antenna system communicationlinks is the elimination or reduction of external interference sources,such as jamming, self-interference, atmospheric noise, man-made noiseand acoustic noise. As is well known, most of the approaches whichattempt to resolve these problems of external interference do so in arelatively complex manner, often times utilizing very large directionalantennas and/or with antennas having hundreds, or even more, elements.This problem of external interference is particularly prevalent in thearea of communication systems where omnidirectional antennas areemployed because of the large number of users operating on the samefrequency band and because of multi-path. One known approach involvesthe use of an antenna configuration comprised of an omnidirectionalantenna in combination with a notch antenna at the receiving end of atransmission link to cancel interference arriving from all directionsexcept over the narrow notch beamwidth or null formed by the notchantenna where the desired signal emanates. By orthogonally combining theantenna signals from the two antennas, all of the undesired interferenceis subtracted from the signal received by the omnidirectional antenna,leaving only the desired signal.

One such system is shown and described, for example, in U.S. Pat. No.4,431,999, entitled, "Interference Cencelling System Using A NotchAntenna And Omnidirectional Antenna", which issued to Frank S. Gutleber,the present inventor, on Feb. 14, 1984. A further illustration of thisconcept is shown and described in U.S. Pat. No. 4,275,379, entitled,"Interference Cancelling Random Access Discrete Address Multiple AccessSystem", which issued to Frank S. Gutleber on June 23, 1981. Theteachings of these patents are specifically incorporated herein byreference.

Accordingly, it is an object of the present invention to provide animprovement in antenna systems having an interference cancellingcapability.

It is another object of the invention to provide an improvement inantenna systems having the capability of orthogonally separating andisolating a plurality of simultaneous interference sources andsubstantially cancelling the signals emanating therefrom.

It is a further object of the invention to provide an antenna systemwhich can cancel or eliminate interference from multiple sources thatare offset from antenna boresight and arrive in the sidelobe region ormainlobe of a large high gain antenna.

It is still another object of the invention to provide an antenna systemwhich provides substantial anti-jam protection for transmission linksthat employ large high gain antennas such as used in a tropospheric,line of sight, or satellite link.

These and other objects are achieved by an approach where adaptiveantenna interference cancelling is obtained by the isolation andindependent tracking of multiple jammers that are received in a sideloberegion of a primary high gain directive antenna. Each of theinterferring signals is orthogonally separated from all of the otherinterferring signals and then it is directly and separately subtractedfrom a composite input signal of the desired signal plus all theinterference signals received by the high gain antenna. Cancelling theinterference does not involve perturbations where the reduction of oneinterferror can result in increasing the signal of another interferror,nor does it require complex adaptive control algorithms.

The interference cancelling concept of the present invention involves amultiplexed interference cancelling system that utilizes a primary highgain antenna and an auxiliary antenna pair including a low gain notchantenna in conjunction with a low gain omnidirectional antenna. Theoutput of these two relatively simple low gain antenna structures aresubtracted from each other to provide an output that is substantiallyfree of all input signals except for the interferring signal received inthe narrow notch of the notch antenna. This signal is then subtractedfrom the composite output signal of the primary antenna. Employing oneof these antenna pairs as an auxiliary antenna pair for each separatejamming signal source to be cancelled enables each jamming signal to beorthogonally isolated, whereupon its magnitude and phase is thenadjusted to be equal to and 180° out of phase with the same componentcontained in the composite signal from the primary antenna. Eachinterfering signal is then separately subtracted from the compositesignal, leaving only the desired signal.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will become readilyapparent when the following detailed description of the invention isconsidered with the accompanying drawings in which:

FIG. 1 is an electrical functional block diagram illustrative of thepreferred embodiment of the invention;

FIG. 2 is an electrical functional block diagram further illustrative ofthe orthogonal signal extractor shown in FIG. 1;

FIG. 3 is a simplified illustration of an antenna pattern of a notchantenna used in the embodiment of the invention shown in FIG. 1; and

FIGS. 4 and 5 are simplified top plan views of an alternative antennapattern and resultant beam processing applicable to a modification ofthe interference cancelling system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, reference numeral 10 in FIG. 1 denotes arelatively large, high gain or narrow beamwidth antenna which receives adesired signal S_(d) in the mainlobe and a plurality(m) of jamming orinterferring signals J₁, J₂ . . . J_(m). The antenna is connected to alike number m of linear signal subtractors 14₁, 14₂ . . . 14_(m). Thesignal subtractors 14₁ . . . 14_(m) are coupled in series to an outputsignal line 15. The signal subtractors 14₁ . . . 14_(m) are also coupledto a respective number of m parallel orthogonal interference signalseparators including m pairs of omnidirectional antennas 16₁, 16₂, . . .16_(m) and notch antennas 18₁, 18₂, . . . 18_(m) as well as a respectivenumber of control loops 20₁, 20₂, . . . 20_(m). Each of the controlloops 20₁, 20₂, . . . 20_(m) includes an orthogonal signal extractor22₁, 22₂, . . . 22_(m) having two respective outputs coupled to athreshold detector 24₁, 24₂, . . . 24_(m) and an output signal gate 26₁,26₂, . . . 26_(m), the latter being controlled by a pulse generator 28₁,28₂, . . . 28_(m) which is triggered by the output of its respectivethreshold detector 24₁, 24₂, . . . 24_(m).

Each of the orthogonal signal extractors includes circuitry shown inFIG. 2 where the ith signal subtractor 22_(i), for example, in additionto including a linear subtractor 30_(i) connected to the omnidirectionalantenna 16_(i), a notch scanner 32_(i) connected to the notch antenna18_(i) and a peak envelope detector 34_(i) as shown in the extractorcircuit 22₁ shown in FIG. 1, also includes separate phase and amplitudeadjusting circuit 36_(i) and 38_(i) coupled between the output of thenotch scanner 32_(i) and the linear subtractor 30_(i) as well as acontrol signal circuit 40_(i) connected between the output of the peakenvelope detector 34_(i) and the notch scanner 32_(i). The output of thepeak envelope detector 34_(i) is also fed to a respective thresholddetector circuit 24_(i) of FIG. 1.

Further as shown in FIG. 2, are top plan view diagrams generallyillustrative of the antenna patterns 42_(i) and 44_(i) of theomnidirectional antenna 16_(i) and the notch antenna 18_(i),respectively. Also shown are the directions from which the desiredsignal S_(d) and a plurality of interfering signals J₁, J₂ and J_(m) areincident on the antennas 16 and 18, as well as the primary 10 shown inFIG. 1.

Each pair of auxiliary antennas 16_(i) and 18_(i) are used to isolateand track one specific jamming signal J_(i) by cancelling all of thesignals except the one that is being tracked. Assuming that i=1 and theposition of the notch 19 in antenna 18_(i) is scanned via the scanner32_(i) until it is pointing in the direction of one of the jammingsignals, e.g. J₁, then the output of the antenna 18_(i) contains thedesired signal S_(d) plus all of the remaining interfering signals, i.e.J₂ and J_(m).

The output of the antenna 18_(i) contains a coherent and correlatedreplica of the desired signal--plus--interference but minus J₁. Thisoutput is then directly subtracted from the signal received by theomnidirectional antenna 16_(i) following its phase and amplitude beingadjusted via the circuits 36_(i) and 38_(i) to provide a coherent andco-related replica of the jamming signal J₁. The subtraction operationis performed in the linear subtractor 30_(i) which as shown is fed tothe peak envelope detector 34_(i) and the threshold detector 24₁. Thepeak envelope detector 34_(i) and the scanner control signal generator40_(i) provide a means for tracking the interfering signal J₁ in a wellknown manner.

The operations associated with each of the auxiliary antenna pairs 16₁,16₂, . . . 16_(m) and 18₁, 18₂, . . . 18_(m) is further clarified asfollows. The resultant signal S_(TN) out of the notch antenna 18_(i) iscomprised of:

    S.sub.TN =J.sub.2 +J.sub.3 . . . +J.sub.m +S.sub.d

where J_(i) is the ith interfering jamming signal and S_(d) is thedesired signal. Therefore, ##EQU1##

The output S_(TO) of the omnidirectional antenna 16_(i) consists of acoherent replica of the same signals as that of the notch antenna plusthe jamming interference signal J₁. Thus: ##EQU2##

The output of the linear subtractor 30_(i) of the ith stage thenprovides an exclusive coherent replica of the jamming interferencesignal J_(i). Where i=1, the J₁ interference signal would be fed fromthe linear subtractor 30₁ to the gate circuit 26₁ of the firstcancelling stage shown in FIG. 1. The gate circuit 26₁ is enabled whenthe output of the threshold detector 24₁ reaches a predeterminedamplitude level, at which time the gate pulse generator 28₁ produces agate pulse that is fed to the gate at 26₁, causing the interferingsignal J₁ to be transferred to a functional block 46₁ and then to thelinear subtractor 14₁.

Signal block 46₁ contains means for adjusting both the amplitude andphase of the signal J₁ similar to the circuits 36_(i) and 38_(i) shownin FIG. 2 to become equal to but opposite in phase to the signal J₁contained in the composite signal S_(TO) out of the receiver 12containing the desired signal S_(d) and all of the interfering signalsJ₁, J₂, . . . J_(m). The amplitude and phase output from the functionalblock 46₁ is further controlled by a null detector 48₁ which is coupledto the output of the linear subtractor 14₁. Thus it can be seen that theamplitude and phase of the interfering signal J_(i) is adjusted in aservo loop context until the output of the null detector issubstantially zero, meaning that J₁ has been removed from the compositesignal from the primary antenna 10.

Each of the succeeding stages 2, 3, . . . m as shown in FIG. 1 operatein the same manner until all the m interfering signals J_(m) are removedfrom the output signal appearing on circuit lead 15.

The notch antenna 18 comprises one of the most important elements in thesubject invention and introduces different requirements than normallyencountered in an antenna design. Instead of involving a configurationwhich forms a directive beam with sidelobes, or utilizing an adaptivesystem with several movable nulls, a fixed pattern which contains auniform reception in all directions except for a narrow beam slot isinvolved. In addition, the slope of the antenna pattern at the point ofthe null is to be as steep as possible. Such a required pattern isillustratively shown in FIG. 3.

General design procedures for providing an array antenna having the typeof pattern shown in FIG. 3 are described in U.S. Pat. Nos. 3,130,410;3,605,106; and 4,580,141. As noted therein, such patterns are made up ofproducts and/or sums of sin mx/sin x functions, and can be achieved bycontrolling both the amplitudes and spacings of array antenna elements.As a result, the slope of a null in an antenna beam pattern can be madesteep, either by providing products of one or more sin mx/sin x terms,or by appropriate amplitude and phase controls when summing several sinmx/sin x functions using subarrays. Further explanation can be had byreferring to a publication entitled, "Coded Linear Array Antenna",published in volume 39, No. 2, of Electrical Communications Magazine.

If some antenna gain is necessary for the auxiliary antennas 16 and 18in order to obtain a received jamming signal level comparable to thelevel arriving in the sidelobe of the primary antenna in addition tobeing at a stronger level than the input noise, then an alternate methodcan be used that is essentially based upon the same principle by using ahigher gain auxiliary antenna that has a steep slope, but with asomewhat wider nulled beamwidth as shown by reference numeral 52 in FIG.4. With this version, the antenna pattern can be electronically scannedto provide a second received beam which is angularly displaced by asmall amount, as shown in FIG. 5. There two received beams 54 and 56 canthen be positioned so that the desired signal J₁ to be orthogonallyisolated, is near the edge of one receiving beam 54, while being nulledout by the second receiving beam 56 as shown. The two received beams 54and 56 would then provide inputs to the linear subtractor 30₁ in amanner analogous to using a notch in an omnidirectional pair. Thisalternate arrangement introduces a second sector (sector B) which wouldprevent isolation of an interfering signal received in section A, if twointerfering sources were simultaneously present in both sector A andsector B. The possibility of this occurring, however, could be madeextremely small by making the sector beamwidth and the non-overlappedregion therebetween extremely small.

The antenna sidelobe cancelling technique described herein can be usedto provide significant ECCM protection in any communication link thatoperates with a high gain, narrow beamwidth antenna such as an LOS,Tropo, or satellite link that operates in the SHF or EHF frequency band.The inventive concept, moreover, can be added directly to an existingsystem as a retrofit that employs a large parabolic dish antenna and hasthe potential of providing a substantial amount of A/J protection.

The primary advantage of the present invention over the known prior artis that it has the capability of orthogonally separating and isolatingmany different sidelobe interferrors simultaneously to facilitateattenuating all of them so that they are substantially totallyeliminated and is accomplished with a relatively simple auxiliaryantenna structure and a nominal amount of signal processing. Inaddition, the response or acquisition time for the proposed system isextremely fast in a relative sense, since the various loops would besimultaneously processing the output of the primary antenna in paralleland the processing does not involve complex, time consuming adaptivecontrol algorithms such as the least mean square (LMS) or powerinversion algorithms.

Thus the orthogonal signal separator for each interference cancellingcontrol loop isolates one of the m different jamming signals whose phaseand amplitude is then adjusted to be of equal strength and oppositesense to the same jamming signal that is present in the composite outputof the main or primary antenna 10. The simple linear subtraction of theadjusted output of the auxiliary antenna pair 16, 18 from the output ofthe main antenna 10 then completely cancels the interference of thespecific isolated jammer. Each of the remaining m-1 jamming signals arecancelled in an identical manner using a separate null tracking loop foreach jamming or interference signal.

Having thus shown and described what is at present considered to be thepreferred embodiment of the invention, it should be noted that the samehas been made by way of illustration and not limitation. Accordingly,all alterations, changes and modifications coming within the spirit andscope of the invention are herein meant to be included.

I claim:
 1. An antenna sidelobe interference cancelling system,comprising:primary receiving antenna means including an output forproviding a first output signal including a desired signal and all of aplurality of interfering signals; at least one auxiliary receivingantenna pair including omindirectional antenna means and steerable notchantenna means, said notch antenna means including a notch in theradiation pattern thereof directed to one of said interfering signals;first circuit means connected to the respective outputs of saidauxiliary antenna pair and differencing the output signals generatedthereby to generate a second output signal corresponding to said oneinterfering signal; and second circuit means connected to the output ofsaid primary antenna means and said first circuit means for subtractingsaid second output signal from said first output signal and generating acontrol output signal.
 2. The system of claim 1 wherein said at leastone auxiliary antenna pair comprises a plurality of auxiliary antennapairs each including respective omnidirectional antenna means andsteerable notch antenna means, said plurality of notch antenna meansbeing directed to selected different ones of said interferingsignals,wherein said first circuit means comprises a like plurality offirst circuit means respectively connected to said auxiliary antennapairs for generating a respective number of second output signals, andwherein said second circuit means subtracts all of said second outputsignals from said first output signal.
 3. The system of claim 2 whereinsaid plurality of auxiliary antenna pairs are at least equal in numberto the number of said plurality of interfering signals.
 4. The system ofclaim 2 wherein said second circuit means includes means for subtractingeach of said second output signals individually from said first outputsignal.
 5. The system of claim 4 and additionally including means foradjusting the amplitude and phase of the respective second outputsignals to match corresponding interfering signals in said first outputsignal from said primary antenna means.
 6. The system of claim 5 andadditionally including means for adjusting the amplitude and phase ofsaid second output signals in response to the control output signal ofsaid second circuit means.
 7. The system of claim 6 wherein said meansfor adjusting the amplitude and phase of said second output signalscomprises null detector means.
 8. The system of claim 4 wherein saidmeans for subtracting each of said second output signals of said secondcircuit means includes a respective number of linear subtractor meansconnected in series for subtracting each of said second output signalsindividually from said first output signal.
 9. The system of claim 4wherein each of said first circuit means further comprises orthogonalsignal extractor means including linear subtractor means including anoutput coupled to the outputs of said auxiliary antenna pair.
 10. Thesystem of claim 9 wherein each said signal extractor means additionallyincludes, notch scanner circuit means coupled to respective said notchantenna means, and peak envelope detector means coupled between theoutput of said linear subtractor means and said notch scanner circuitmeans for causing the notch in the radiation pattern of said respectiveantenna means to track a respective one of said interfering signals. 11.The system of claim 10 wherein each said signal extractor meansadditionally includes signal amplitude and phase adjusting means coupledbetween the output said respective notch antenna means and said linearsubtractor for causing the output of said respective notch antenna meansto match the output of said omnidirectional antenna.
 12. The system ofclaim 10 and additionally including signal gate circuit means coupledbetween each said linear subtractor means and each said second circuitmeans, and means responsive to the amplitude of the output of saidrespective notch antenna means for enabling said gate circuit means whensaid output reaches a predetermined amplitude.
 13. The system of claim12 wherein said means responsive to amplitude comprises a thresholddetector.
 14. The system of claim 13 and additionally including gatepulse generator means coupled between said threshold detector and saidgate circuit means for generating an enabling pulse for said gatecircuit means in response to the output of said threshold detector. 15.The system of claim 14 and additionally including means for adjustingthe amplitude and phase of the respective second output signals to matchcorresponding interfering signals in said first output signal from saidprimary antenna means.